Strings for racquets

ABSTRACT

Novel racquet strings and methods for making the same. A polymer cover combined with a low temperature adhesive is provided to the strings. The string of the present invention may employ a conventional string, such as a string having a center core comprising gut or synthetic material such as nylon, and a polymer cover impregnated with low temperature adhesive. The polymer cover covers the string along at least a portion of the length of the string.

BACKGROUND

The present invention relates to strings for sporting applications, andparticularly to strings for racquets such as tennis racquets, badmintonracquets, squash racquets, racquetball racquets and the like.

Racquet strings must satisfy competing requirements. In a tennisracquet, for example, the principal requirements are playability anddurability and it is difficult to satisfy both requirements in a singleracquet string type. String construction and material selection hasheretofore required a compromise between acceptable playability anddurability.

During play, particularly in tennis, the ball is usually hit with somedegree of spin. To generate spin, the strings are brushed against theball to impart a tangential force to it. This brushing action causes theindividual strings to slide over one another and wear against eachother. The rubbing action of one string against another as well as theimpact of the ball creates notches in the strings at the inter-stringcontact point. These notches are the primary reason for string breakage;as the notching becomes more severe, the tensile strength of the stringweakens and eventually it breaks. The friction between the ball and thestring during contact with the string surface also causes some stringwear.

Several materials have been used in racquet strings in order to achievea balance of durability and playability. One material, natural gut,enjoys a reputation for unmatched playability. Unfortunately, gutstrings have a short life due to notching and wear. Few recreationalplayers use gut strings because they are expensive and wear quickly.Many synthetic string materials, sizes and constructions have beenproposed as alternatives to gut. Such synthetics generally are moredurable than gut, but are not as playable.

The most common synthetic material is nylon. Although more modernfibers, such as PEEK and aramid fibers, such as Kevlar® are used inracquet strings, nylon multi-filament strings are generally accepted asamong the most playable synthetic materials. Nylon strings demonstrateimproved durability over gut, but even nylon strings are subject tofrequent breakage by certain players, particularly power hitters andthose who hit the ball with a lot of spin.

Nylon strings have been proposed in many mono-filament andmulti-filament constructions, the more durable strings being themono-filaments and the more playable strings being the multi-filaments.Within the range of mono-filaments and multi-filaments there are avariety of constructions that have been used to either tailor thedurability or the playability of the string.

Coatings have been proposed to improve the abrasion resistance ofstrings. For example, strings have been dipped or coated withpolytetrafluoroethylene in an attempt to reduce the friction betweenstrings, which causes notching. Other attempts have included addinghard, abrasion resistant coatings to the exterior of a string. Suchcoatings have generally failed because they are inelastic and do notadhere well to the nylon surface as the string stretches in use.

To improve the durability of nylon strings, the addition of highstrength fibers such as aramids to multi-filament constructions has beenproposed. However, the addition of stiff aramid fibers to a stringmatrix dramatically reduces the playability of the strings. Nomex, whichhas better elastic properties than other aramids, has been added to thecore of nylon strings with some success with regards to durability, butwith a significant tradeoff with regard to playability.

An accepted measure of playabiltiy is dynamic modulus, which is theratio between the increase of tension and the elongation of a stringcaused by dynamic impact. This is a measure of how stiff a string isunder dynamic conditions similar to that of being struck with a tennisball during play. To be playable, a racquet string must show elasticproperties under dynamic conditions and deform under a given impact.Strings with low dynamic modulus are less stiff and therefore havebetter playability than strings with a high dynamic modulus, which donot stretch as much and therefore feel stiff. Gut strings may have adynamic modulus of as low as 17-26 kN/m. In contrast, high strengthfibers such as Kevlar may have a dynamic modulus of 88 kN/m to 140 kN/mor more. Nylon strings have a dynamic modulus in a range of about 25kN/m to about 45 kN/m.

Another factor affecting both durability and playabiltiy is string size,or gauge. For example, a 16 gauge string generally has a larger diameterthan a 17 gauge string. Accordingly, the 16 gauge string may lastlonger. But string size is critical to playability, and thinner stringsplay better.

Higher gauge strings or thinner diameter strings play better in partbecause they are more effective at imparting spin to a ball, such as atennis ball, because thin strings cut deeply into the felt cover of atennis ball, gripping it to impart the spin necessary for playercontrol. Thicker strings do not penetrate the ball cover as deeply.Thinner strings also deflect more for a given impact. This increase indeflection reduces the shock that the player feels and returns moreenergy to the ball giving the player more power. Furthermore, thickstrings increase wind resistance to racquet swing to a surprisingdegree.

While the dynamic modulus of an individual string is indicative of itsplayability, racquets are actually strung with a crossed pattern ofstrings called a string bed. The strings extending from top to bottom ofthe racquet head are called the main strings, while those crossing theracquet head are called the cross strings. When the strings move withinthe string bed, the main strings slide and rub against the crossstrings. The resultant friction between strings causes energy loss. Thisenergy loss may also affect playability.

SUMMARY

The present invention includes improved strings for racquets and methodsfor making the same.

The string of the present invention may employ a conventional string,such as a string having a center core comprising gut or syntheticmaterial such as nylon, and a polymer cover impregnated with adhesive.The adhesive may be low temperature adhesive. The polymer cover coversthe string along at least a portion of the length of the string. As theterm “adhesive” is used herein it is intended to mean a material thatwill form a bond between the polymer cover and the base string. As theterm “low temperature adhesive” is used herein it is intended todesignate any adhesive that will either form a bond when processed at atemperature less than about 300° C. More preferably, the low temperatureadhesive comprises any adhesive that will either cure or form a durablebond at less than about 275, 250, 225, 200, 175, 150, 125, 100, 75, 50,or 25° C.

In one aspect, the invention provides a string, and a compositecomprising a polymeric membrane having at least some porosity and anadhesive disposed within the at least some porosity, the compositecovering at least a portion of the string.

In another aspect, the polymer cover has at least some porosity. Inanother aspect of the invention, at least some of the porosity is filledwith an adhesive by applying the adhesive to one or more surfaces of thepolymer cover. In an alternative embodiment of the invention, at leastsome of the porosity is filled, for example, by imbibing or impregnatingthe porous polymer cover, with adhesive.

In a still further aspect, the adhesive is applied to at least onesurface of the polymer cover and at least some of the porosity is filledwith an adhesive.

In an alternative embodiment of the invention, the adhesive is a lowtemperature adhesive.

In yet another aspect of the invention, a suitable low temperatureadhesive can be applied to at least one surface of the polymer cover andthe low temperature adhesive may form a durable bond between the stringand cover material. In this aspect, the adhesive may be continuous ordiscontinuous.

In order to provide the highest compatibility with a wide variety ofunderlying string materials, it may be desirable to provide an adhesivematerial that can be applied, and if necessary cured, at or near roomtemperature, such as through use of pressure sensitive adhesives,radiation curable adhesives, or the like. Thus, in another aspect of theinvention an adhesive is provided that is cured through exposure toultraviolet light (hereinafter “UV” light) or an electron beam(hereinafter “EB”).

In yet another aspect of the invention, the polymer cover comprisesePTFE.

In another aspect, the composite has a thickness of less than about 5%of the racquet string diameter. Preferably, the composite has athickness of less than about 3% of the racquet string diameter. Mostpreferably, the composite has a thickness of less than about 1% of theracquet string diameter.

In still another aspect, the string is of monofiliment construction.Preferably, the string is of multifilament construction. In this aspect,the filaments of substantially the same diameter are preferred.

In another aspect, the racquet string of the present invention includesa base string constructed of Nylon, PEEK or gut.

In a still further aspect, the invention provides for an adhesivecomprising at least one filler material. In this aspect, the fillermaterial may be selected from the group consisting of ceramics, metals,metal coated fillers, metallized fillers, inorganic oxides, carbon,pigments, lubricants and polymers.

In another aspect, the adhesive comprises a urethane acrylate or acationic epoxy.

In yet another aspect of the invention, the cover is helically wrappedaround the base string.

In another aspect the invention is a racquet string having a diameter ofless than about 1.34 mm and having dynamic modulus of less than about 30kN/m and a durability of at least about 2200.

In a still further aspect, the invention is a racquet string having adiameter of less than about 1.25 mm and having dynamic modulus of lessthan about 30 kN/m and a durability of at least about 500 and at leastabout 1000.

In a yet another aspect, the invention is a racquet string having adiameter of less than about 1.20 mm and having dynamic modulus of lessthan about 30 kN/m and a durability of at least about 500 and at leastabout 800.

DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from thefollowing description when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is a three-quarter perspective view of a racquet;

FIG. 2 is a schematic drawing of a porous film of the invention whereinat least some of the porosity of the film is filled with adhesive;

FIG. 3 is a schematic drawing of a porous film of the invention whereinsubstantially all of the porosity of the film is filled with adhesive;

FIG. 4 is a schematic drawing of a porous film of the invention whereinat least some of the porosity of the film is filled with adhesive andone surface of the film is provided with a relatively thin layer ofadhesive;

FIG. 5 is a schematic drawing of a porous film of the invention whereinsubstantially all of the porosity of the film is filled with adhesiveand one surface of the film is provided with a relatively thin layer ofadhesive;

FIG. 6 is a schematic drawing of a porous film of the invention whereinsubstantially all of the porosity of the film is filled with adhesiveand both surfaces of the film are provided with a relatively thin layerof adhesive;

FIG. 7 is a schematic drawing of a porous film of the invention whereinat least some of the porosity of the film is filled with adhesive, butthe adhesive is not coincident with the surfaces of the film;

FIGS. 8 a through 9 b show string constructions according to theinvention.

FIG. 10 is a perspective view of the apparatus used to determine stringdurability.

FIG. 11 is a schematic diagram of the apparatus used to determineelastic modulus of strings.

FIG. 12 is a perspective view of the apparatus used to determine theelastic modulus of strings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to improved racquet strings.

The invention solves the problem of string durability withoutdiminishing the playability of the string. This is accomplished bywrapping (or otherwise covering) at least a portion of the string with apolymer cover The cover should be sufficiently durable to withstandabrasion occasioned by ball impact and string movement.

The polymer cover has at least some porosity. As used herein, “porosity”refers to the property or state of a material having voids orinterstices. The cover may be impregnated with an adhesive by applyingadhesive to one or more surfaces of the polymer cover. By utilizing apolymer cover comprising at least some porosity, at least some of theporosity is filled with the adhesive.

This novel construction uniquely combines the hardness of a durableadhesive with the lubricity of a polymer having a low coefficient offriction. A polymer string cover alone may not provide adequate abrasionresistance; however, the inventors have found that by filling at leastsome porosity of the polymer cover with adhesive, abrasion resistancemay be significantly improved, while playability is retained. In thisway, the porous polymer provides a lubricious matrix that supports thehighly abrasion resistant adhesive. The abrasive resistant adhesive isbound within this matrix and, despite its hardness, does not flake fromthe flexible string. Moreover, the lubricious polymeric membrane reducesfriction between strings. Reducing friction at the intersection ofstrings may further improve playability by improving energy return, andfurther reduces string breakage by inhibiting notches.

In an aspect of the invention, a suitable low temperature adhesive canbe applied to at least one surface of the polymer cover and the lowtemperature adhesive may form a durable bond between the string andcover material.

The porous polymer cover of the present invention improves durability byproviding a wear-resistant surface, but avoids the problem ofrestricting elongation or movement of the string. Moreover, by fillingat least some, or substantially all, of the porosity of the cover withadhesive, durability of the string is further improved.

It has been discovered that the porous polymer can be altered towithstand substantial wear and abrasion during use. Wear and abrasionresistance can be improved by, for example, careful selection of theadhesive used, the addition of certain filler materials, as well as theamount of porosity filled with the adhesive. Thus, by careful selectionof adhesive type, amount of adhesive used, and filler materials (ifused), an extremely durable and abrasion resistant cover can befabricated to inhibit notching.

The present invention also solves the problem of string contamination.In applications such as tennis, grass and clay courts in particular mayexpose the string to contaminants. The polymer cover protects the corefrom abrasive contaminants that contribute to premature wear, such asclay and silica which are transferred from the court surface to thestring by ball impact.

Materials suitable for use in the porous polymer cover of the presentinvention include, but are not limited to, the following fluoropolymers:polytetrafluoroethylene (PTFE), particularly porous expanded PTFE(ePTFE); fluorinated ethylene propylene (FEP); polyethylene, includingultrahigh molecular weight polyethylene; perfluoro alkoxy resin (PFA);polyurethane; polypropylene; polyester; polyimide; and polyamide.

Although the invention includes use of any porous polymer covermaterials, particularly preferred are porous fluoropolymer films, withPTFE and ePTFE being even more preferred. The porosity of the porouspolymer cover can be either partially or substantially filled withadhesive. For example, a relatively small amount of adhesive can besupplied to a select portion of the film porosity, while leaving most ofthe porosity of the film unfilled. In an aspect of the invention,adhesive can be evenly distributed throughout the porosity of the coverfrom one side of the cover to the other side, while still leaving atleast some porosity unfilled. Moreover, in a further aspect of theinvention, substantially all of the porosity of the film can be filledwith adhesive to perhaps result in better abrasion resistance and betteradhesion.

Turning to the figures, FIG. 1 shows a tennis racquet with main strings(20) and cross strings (22). FIG. 2 illustrates a porous cover material1, where at least some of the porosity 2 is filled with adhesive 3. FIG.3 illustrates a porous cover where substantially all of the porosity 2is filled with adhesive 3. FIG. 4 illustrates an aspect of the inventionwherein at least some of the porosity 2 is filled with adhesive 3 and anadditional surface layer of adhesive 4 is supplied to one surface of thefilm. FIG. 5 illustrates an aspect of the invention where substantiallyall of the porosity 2 has been filled with adhesive 3 and an additionalsurface layer of adhesive 4 is supplied to one surface of the film. FIG.6 illustrates an aspect of the invention where substantially all of theporosity 2 has been filled with adhesive 3 and both surfaces of thecover are supplied with a surface layer of adhesive 4 and 5. FIG. 7illustrates an embodiment in which some of the porosity 2 is filled withadhesive 2 and both sides of the film are supplied with a surface layerof adhesive. Although covers with any amount of porosity may be used,preferably the cover has a bulk density about 0.7 g/cc, before fillingwith adhesive.

A preferred cover material is a porous fluoropolymer material such asuniaxially expanded PTFE. This material has demonstrated exceptionaldurability without affecting the playability of the base string. Porousexpanded PTFE, such as that made in accordance with U.S. Pat. Nos.3,953,566; 3,962,153; 4,096,227; and 4,187,390, comprises a porousnetwork of polymeric nodes and interconnecting fibrils. These kinds ofmaterial are commercially available in a variety of forms from W. L.Gore & Associates, Inc., Newark, Del.

Expanded PTFE is formed when PTFE is heated and rapidly expanded bystretching in at least one direction in the manner described in theabove listed patents. The resulting expanded PTFE material achieves anumber of exceptional properties, including exceptional strength in thedirection of expansion, and exceptionally high flexibility, andconformability. The strength properties in both the longitudinal andtransverse directions of the ePTFE may be altered in the expansionprocess, or by other means known in the art to achieve the desiredeffect or property.

As the term “expanded PTFE” is used herein, it is intended to includeany PTFE material having a node and fibril structure, including in therange from a slightly expanded structure having fibrils extending fromrelatively large nodes of polymeric material, to an extremely expandedstructure having very long fibrils interconnected by small nodes. Thefibrillar character of the structure is identified by microscopy. Whilethe nodes may easily be identified for some structures, many extremelyexpanded structures consist almost exclusively of fibrils with verysmall nodes.

When a porous polymer cover material is used, at least some, orsubstantially all, of the porosity of the porous polymer cover can befilled with adhesive. Additionally, adhesive can also be provided as acontinuous or discontinuous coating on one or both sides of the cover.As used herein, “discontinuous” means that the adhesive does not fullycover the surface of the underlying cover. “Continuous” means withoutholes or gaps extending through the adhesive coating (i.e. fullycovering the surface of the underlying cover). The exact amount ofadhesive used depends upon a number of variables. For example, addingmore adhesive may further improve durability and abrasion resistance,but may also increase string mass, which may affect playability.Providing less adhesive may result in less durability and reducedabrasion resistance. However, less adhesive may tend to preserveplayability. Once the cover has been impregnated or otherwise filledwith adhesive, the preferred percent mass of adhesive to ePTFE is 45%.

In order to provide the highest compatibility with a wide variety ofunderlying string materials, it may be desirable to provide a lowtemperature adhesive that can be applied, and if necessary cured, at ornear room temperature, such as through use of pressure sensitiveadhesives or radiation curable adhesives, or the like.

Low temperature adhesives include any adhesive that will either cure orform a durable bond when processed at a temperature of less than about300° C. Suitable low temperature adhesives include any suitablethermoset resin. For example, suitable thermoset resins include epoxies(including acrylated epoxies), polyurethanes, phenolics, and otherthermosets. Suitable thermoplastic resins include, for example,polyethylene, polypropylene, polystyrene, polyvinyl chloride,polyurethanes, and fluoropolymers such as THV (tetrafluoroethylene,hexafluoropropylene, and vinylide fluoride), HTE (hexafluoropropylene,tetrafluoroethylene, and ethylene), EFEP (ethylene tetra fluoro ethylenebased copolymer), ETFE (ethylene tetrafluoroethylene), and PVDF(polyvinylidine fluoride), and blends thereof. Other thermoplasticresins are also useful, provided that they are processable attemperatures of less than about 300 C.

Thermally activated adhesives which can cure or form a durable bond whenthe adhesive is heated, such as THV 220 (tetrafluoroethylene,hexafluoropropylene, and vinylide fluoride, available from Dyneon, LLC)and adhesives which can be caused to cure through chemical reaction,such as known moisture cure adhesives (e.g., polyurethane prepolymers,etc.) or other chemically activated adhesives, can also be used.

In a preferred embodiment, the low temperature adhesive comprisesUV-curable adhesive. As used herein, UV-curable is defined as a materialthat will react under UV light to cure or form a durable bond. The UVlight is provided by a lamp with suitable spectral intensity, spectraldosage and wavelength. Those of skill in the art will appreciate thatcuring with UV light may be carried out at various rates, and that thedistance between the sample being cured and the UV lamp can be varied,provided the appropriate spectral dosage is applied. In an aspect of theinvention, the UV curable material can also be sensitive to visiblelight. However, preferred conditions are present only under the UVspectrum (100-400 nm). In this range, the underlying core material willnot be damaged during the processing of the string. Suitable UV-curableadhesives include, but are not limited to, epoxies, acrylated epoxies,acrylated urethanes, acrylated silicones, acrylated polyethers,acrylated polyester, acrylated polybutadiene, and acrylatedfluoropolymers. Specific examples of these adhesives include acrylatedaliphatic oligomers, acrylated aromatic oligomers, acrylated epoxymonomers, acrylated epoxy oligomers, aliphatic epoxy acrylates,aliphatic urethane acrylates, aliphatic urethane methacrylates, allylmethacrylate, amine-modified oligoether acrylates, amine-modifiedpolyether acrylates, aromatic acid acrylate, aromatic epoxy acrylates,aromatic urethane methacrylates, butylene glycol acrylate, stearylacrylate, cycloaliphatic epoxides, cylcohexyl methacrylate, ethyleneglycol dimethacrylate, epoxy methacrylates, epoxy soy bean acrylates,glycidyl methacrylate, hexanediol dimethacrylate, isodecyl acrylate,isooctyl acrylate, oligoether acrylates, polybutadiene diacrylate,polyester acrylate monomers, polyester acrylate oligomers, polyethyleneglycol dimethacrylate, stearyl methacrylate, triethylene glycoldiacetate, and vinyl ethers. Preferred UV-curable adhesives include, forexample, urethane acrylates and cationic epoxies.

It may be desirable to utilize a solvent to aid in providing adhesive tothe porosity of the porous polymer cover. The ratio of solvent materialto adhesive can vary and will also be readily determinable by theskilled artisan. Preferable solvent materials will also be apparent toone skilled in the art and include, for example, alcohols, ketones, etc.A preferred solvent is isopropyl alcohol (IPA). When a solvent materialis utilized, the solvent material can be easily removed or driven offonce the adhesive is provided to at least some of the porosity of theporous polymer cover as desired.

In a further aspect of the invention, the low temperature adhesive canbe combined (e.g., mixed, blended, etc.) with a suitable fillermaterial. Suitable filler materials may include, but are not limited to,ceramics, metals, inorganic oxides, metal coated materials, metallizedmaterials, carbon, pigments and polymers, which can be provided in anysuitable form (e.g., particulates, fibers, etc.) Preferably, fillers arein nanoparticle size. Filler materials may be desirable to alter certainproperties of the covered string (e.g., to improve abrasion resistance,or to provide color, etc.). Use of solvent may be particularly usefulwhen at least partially filling the porosity of a porous cover with anadhesive/filler material combination.

The adhesive may be applied to the cover by a variety of methods knownin the art. With regard to porous polymer covers, suitable adhesiveapplication means include, for example, coating techniques (e.g., dipcoating or spray coating), solvent imbibing, vacuum assisted coating,pressure assisted coating, nip coating, and other suitable means whichwould result in the adhesive filling at least some of the porosity ofthe porous polymer cover.

As stated above, a preferred porous polymer cover is expanded PTFE. Atleast a portion of the porosity of the expanded PTFE is filled with lowtemperature adhesive. In an aspect of the invention, substantially allof the porosity of the expanded PTFE film is filled with low temperatureadhesive. Furthermore, one or more surfaces of the expanded PTFE may beprovided with a relatively thin surface layer of low temperatureadhesive for bonding the cover to the base string. Such surface layer(s)of adhesive can be either continuous or discontinuous. In a preferredembodiment the surface layer(s) of adhesive is a continuous layer.Preferably, the film is impregnated with an adhesive/solvent solution,thus allowing good penetration of the adhesive into the porosity of thefilm. Impregnating is accomplished by first preparing anadhesive/solvent solution, and second, combining this solution with aporous film like expanded PTFE. Solvents such as alcohols and ketonesare capable of dissolving adhesives so that the adhesive can penetrateand occupy the porosity of the porous film. There are many suitableadhesives (e.g., urethanes, epoxies, etc.) that can be dissolved insuitable solvents. In an aspect of the invention, the adhesive isUV-curable urethane-acrylate. This adhesive will also cure by othermechanisms such as through heating and chemical reaction.

The mass of adhesive delivered to the expanded PTFE film (or otherpolymer cover material) can be regulated by the solvent to adhesiveratio in the solvent/adhesive solution and by the rate at which it isapplied. A spreading mechanism can be used to distribute theadhesive/solvent solution after it contacts the film surface. Once thefilm has accepted the adhesive/solvent solution, or becomes impregnated,the mechanical characteristics of the film can change and it may havethe tendency to shrink. In order to stabilize the film, a suitable linercan be provided to the film following this step. An example of asuitable liner material is polyester release film. Another suitableliner material may be a silicone-coated paper. In any event, both theliner and the film can be contacted together and placed into a forcedair oven. The heated air can be blown across the flat side of the filmoriented with the non-liner side toward the air stream. This drives offthe solvent and leaves the adhesive within the porosity of the film. Thefilm can be removed from the liner before applying the film to thestring.

Once the low temperature adhesive has been provided to at least onesurface of the polymer cover, and the low temperature adhesive has atleast partially filled, or is otherwise provided to, the porosity of thecover (and the solvent driven off, if a solvent is used), the cover canthen be placed in contact with the string and the low temperatureadhesive can then be cured.

The cover of the present invention may be applied in a variety ofmanners while maintaining the benefits of the present invention. Thecover may be wrapped longitudinally (in a “cigarette wrap” manner), oras a continuous and seamless tube surrounding the string. Preferably,the string is helically wrapped with a cover material. In thisembodiment, the string may be provided with a cover in the form of awrapped polymer layer having overlapping edges to form a continuouscover or with non-overlapping edges. The polymer layer may optionally beheated to thermally bond the overlapped edges together. The cover may ormay not include an adhesive coating on its outwardly facing surface. Theadhesive coating serves to adhere the wraps to the base string and mayalso provide an additional protective layer to shield the cover fromwear and contamination.

Although particularly preferred base string materials include gut ornylon materials, cores comprising other synthetic materials or aramidfibers may also benefit from the use of covers made and applied inaccordance with the present invention. However, the covers areparticularly attractive when used in combination with highly playablestrings, such as nylon or gut strings. Although gut and nylon aretypical materials for strings, another preferred material for the stringof the invention is PEEK. PEEK strings may provide better durabilitythan nylon and demonstrate acceptable playability.

Regardless of the type of base string, once the string is provided withthe cover, the adhesive can be cured to result in the covered string ofthe invention.

The particular curing mechanism used, such as heat, UV/EB radiation, andchemical reaction, will depend on the type of adhesive used. Onepreferred adhesive is urethane-acrylate, which is capable of curing viaheating and/or UV radiation. The preferred mechanism for curing thisadhesive on a gut or synthetic string is UV radiation because of itsrelatively low temperature application.

As discussed above, prolonged high temperature processes can degrade theperformance of strings with gut or synthetic components by compromisingthe properties of the materials therein. Degraded performance may beobserved as a reduction of durability or an increase in dynamic modulus.It is therefore desirable to process strings at temperatures that do notchange string performance. Thus, in an aspect of the invention preferredlow temperature adhesives include adhesives that bond or can be cured ata temperature of about 150° C. or less and, in a further aspect of theinvention, at a temperature of about 120° C. or less. More preferably,the low temperature adhesive comprises any adhesive that will eithercure or form a durable bond at less than about 100, 75, 50, or 25° C.

To cure the adhesive by UV/EB radiation, the covered string can beplaced in tension to keep the covered string straight. Importantparameters for the UV curing process are spectral intensity of UV light,measured by Wafts/cm², and spectral dosage of UV light, measured byJoules/cm². The preferred light intensity, wavelength and dosage dependupon the selection of photoinitiators and the formulation of theadhesive blend, and are readily determined by one of skill in the art.Upon exiting the UV oven, the string should have a tack free surface,indicating that the adhesive has cured.

In an aspect of the invention, a single layer of expanded PTFE, havingbeen stretched in the longitudinal and transverse directions andimpregnated with adhesive, is provided to the base string. This isaccomplished by helically wrapping the string at a pitch angle measuredfrom the end of the string. This construction is believed to provideexcellent strength and durability while maintaining the playability ofthe base string.

Without intending to limit the scope of the present invention, thefollowing examples illustrate how the present invention may be made andused:

EXAMPLES Example 1

An example of a string according to the present invention was preparedby helically wrapping a 1.19 mm diameter multi-filament nylon stringthat was obtained from Prince Mfg. Co. with a polymer film impregnatedwith UV-curable adhesive. The string was made in the following manner:

Expanded PTFE film with a thickness of about 0.015 mm was obtained fromWL Gore and Associates, Inc., Newark, Del. The expanded PTFE film had abulk density of 0.7 g/cc, and was further characterized by a matrixtensile strength of about 41,000 psi in the longitudinal direction and aBubble Point of 68 psi.

A 30 wt. % adhesive solution was prepared in isopropyl alcohol forimpregnating the expanded PTFE film. The adhesive composition is 60 wt.% aliphatic polyester based urethane diacrylate oligomer blended withethoxylated trimethylol propane triacrylate (available from SartomerCompany, Exton, Pa. as CN963E75), 32 wt. % triacrylate acid ester(available from Sartomer Company as CD9052), and 8 wt. % Genocure DMHA,available from Rahn USA Corp., Aurora, Ind. This solvent-adhesivesolution was dispensed and spread evenly across the expanded PTFE film.A polyester release film grade UV5010 was used as a liner and combinedwith the film as the solvent-adhesive solution penetrated the expandedPTFE film. Both the liner and impregnated film were sent through an oven(set at about 120° C.) to drive off the solvent. The film was removedfrom the oven and a substantially fully impregnated structure withadhesive coincident with both surfaces of the film and a thin surfacecoat of adhesive present on the liner side was recovered. The thinsurface coat substantially covered the expanded PTFE surface.

The 3.56 mm wide impregnated film was wrapped in a non-overlappinghelical fashion around the base string at a pitch angle of 32 degreeswhile leaving little or no gap between the film layers contacted. Theresultant construction was a string with a single layer of impregnatedfilm covering the entire length of the string.

The covered string was fed through a 300 Watt F300S Electrode-less UVLamp System provided by Fusion UV Systems, Inc., Gaithersburg, Md. TheUV lamp was equipped with an H-bulb and F6 light shield for wire/cableapplications with 360° reflection. UV dosage to cure the adhesive wascontrolled by the line speed, which was set to 20 ft/min. Prior toinserting the string, the UV oven was purged with nitrogen to removeoxygen from the oven.

Once each string exited the UV lamp system, it was observed to have atack-free surface, indicating that the impregnated adhesive had cured.It was further noted that the cover conformed to the string. The coveredstring diameter was 1.24 mm.

The string was installed in a racquet and was found to have excellentplayability (that is, the playability was at least equal to that ofcomparable diameter multifilament nylon strings as measured by thedynamic modulus). During play, the strings felt smoother and did notrequire repositioning as frequently as an uncovered string.

Moreover, the durability was significantly improved. During play tests,the strings exhibited noticeably less notching at string contact points.The covered string was also tested on a durability tester, and thedurability was reported in Table 1 below. The inventive strings showsignificant durability improvement over a comparable diameter nylonstring, without increasing the dynamic modulus.

Example 2

A second, slightly smaller example string was made according to thepresent invention by helically wrapping a 1.13 mm diametermulti-filament nylon string that was obtained from Prince Mfg. Co. witha polymer film impregnated with UV-curable adhesive. The string was madein the manner described above in Example 1 by wrapping the base stringwith the same ePTFE film impregnated with a low temperature adhesiveused in Example 1.

The final string diameter was 1.18 mm. As reflected in Table 1, theinventive string shows much better durability than a comparable diameternylon string, and a similar dynamic modulus.

Test Methods

Durability

The durability test apparatus is depicted in FIG. 10. Tennis balls werealternatively fired at 60 MPH from two ball machines 15, 15′ such thatthe balls contacted a simulated racquet frame 17 at a rate of one every4 seconds. Ball speed was measured at each ball machine using laserspeed recording equipment. The simulated racquet string bed 30 wasconstructed of an 8¾″×11½″ rectangular aluminum frame. The string bedwas strung with a pattern of 16 main strings 32 and 18 cross strings 34.Delrin® grommets (not shown) were used to reduce string wear at theframe. The frame was strung at a tension of 58 pounds. The frame wasplaced perpendicular to the ground such that the discharges 36 of theball machines were both 25″ from the center of the sting bed. The ballstraveled along flight path 38 and contacted the string bed at ahorizontal angle of 60° and an upward vertical angle of 15°. This wasintended to simulate top-spin action.

The test was conducted at an ambient temperature of 20° C. Ten newTretorn Micro X 90 balls were loaded into the ball machines. As eachball machine alternately fired the balls at the string bed, the mainstrings moved back and forth against the cross strings. After impact,the balls were continuously fed back into the ball machines. The ballswere continuously fired from both machines until a string broke.Durability was measured and recorded as the number of impacts at whichthe string broke.

Dynamic Modulus

An apparatus for testing the dynamic modulus of a string is depicted inFIG. 12 and schematically illustrated in FIG. 11. A string sample 40 washeld horizontally in metal clamps 42, 42′ spaced 340 mm (12.6 in) apart.Two metal bars 44, 44′ were positioned between the clamps justcontacting the string to support the string. The bar centerline spacingwas 300 mm. The string was tensioned at 28 kg. The test was conducted atan ambient temperature of 20° C.

A pendulum 46 was swung into the string to contact the string at thecenterpoint of the span between the support bars. The pendulum includesa 0.8″ (20.3 mm) flat head hammer face 48, which makes contact with thestring. The pendulum weighs 720 g, and its barycenter is 450 mm from therotation point. At impact, the angular speed is 5.35 rad/s, resulting ina hammer speed of 3.18 m/s.

When the hammer face hits the string, the maximum deflection of thecenter point of the string was measured using laser measurementequipment. The maximum tension increase in the string was also monitoredusing a load cell attached to one end of the string. From the maximumdeflection (D_(max)) and span L_(Orig), the total lengthwise stretch(ΔL) was calculated according to the formula:ΔL=L _(Max) −L _(Orig)where L_(orig) is the original string length and Lmax is the maximumstring length. Maximum string length is determined by the equation:$L_{Max} = {2 \star \sqrt{\left( \frac{L_{Orig}}{2} \right)^{2} + D_{Max}^{2}}}$Where D_(max) is the maximum deflectionThe dynamic modulus, k, may be calculated by dividing the maximum changein string tension (ΔT) as measured by the load cell by the totallengthwise stretch (ΔL) at impact. Dynamic modulus has units of kN/m.Bubble Point

The Bubble Point test provides an estimation of maximum pore size.Liquids with surface free energies less than that of stretched porousPTFE can be forced out of the structure with the application of adifferential pressure. This clearing will occur from the largestpassageways first. A passageway is then created through which bulk airflow can take place. The air flow appears as a steady stream of smallbubbles through the liquid layer on top of the sample. The pressure atwhich the first bulk air flow takes place is called the bubble point andis dependent on the surface tension of the test fluid and the size ofthe largest opening.

The Bubble Point is measured using the procedures of ASTM F316-86 asguideline. Isopropanol was used as the wetting fluid to fill the poresof the test specimen. The test sample is placed in a filter holder(available from Millipore Corporation, Billerica, Mass.), covered with asupport screen and the locking ring of the holder attached. The top ofthe holder is then filled with isopropanol, and the holder is attachedto an air supply with a regulated control valve. The holder is placedunder a magnifying lens with a light and the air pressure is increaseduntil a continuous stream of bubbles is seen coming through the supportscreen covered with isopropanol.

The Bubble Point is the pressure of air required to displace theisopropanol from the largest pores of the test specimen and create thefirst continuous stream of bubbles detectable by their rise through alayer of isopropanol covering the porous media.

Matrix Tensile Strength

Tensile strength of ePTFE materials including ePTFE films is measuredusing an INSTRON tensile testing machine with pneumatic cord and yarngrip jaws. The machine tested 0.25 inch wide samples using a 1 inch jawseparation distance and a crosshead speed of 10 inches/minute. Matrixtensile strength of porous PTFE samples is determined by the formula:(2.2 g/cc×tensile strength)/density of tested material, where 2.2 g/ccis taken to

be the density of non-porous PTFE. TABLE 1 Durability Dynamic ModulusString Diameter (Impacts) (kN/m) Comparative 1.24 mm 398 29.66 Example 1Inventive 1.24 mm 1163  25.65 Example 1 Comparative 1.19 mm 344 25.76Example 2 Inventive 1.18 mm 910 24.99 Example 2

1. A racquet string comprising: a) a string; and b) a compositecomprising a polymeric membrane having at least some porosity and anadhesive disposed within said at least some porosity, the compositecovering at least a portion of said string.
 2. The racquet string ofclaim 1 wherein said at least some porosity is filled with the adhesive.3. The racquet string of claim 2 further comprising an adhesive layerdisposed upon at least one surface of said polymeric membrane.
 4. Theracquet string of claim 3 wherein the adhesive layer is discontinuous.5. The racquet string of claim 3 wherein the adhesive layer iscontinuous.
 6. The racquet string of claim 1 in which the adhesivecomprises a low temperature adhesive.
 7. The racquet string of claim 6in which the low temperature adhesive is UV cured.
 8. The racquet stringof claim 6 wherein the low temperature adhesive further comprises atleast one filler material.
 9. The racquet string of claim 8 wherein theat least one filler material comprises at least a material selected fromthe group consisting of: ceramics, metals, metal coated fillers,metallized fillers, inorganic oxides, carbon, pigments, lubricants andpolymers.
 10. The racquet string of claim 6 wherein the low temperatureadhesive comprises at least a material selected from the groupconsisting of urethane acrylates and cationic epoxies.
 11. The racquetstring of claim 1 in which the composite is helically wrapped around atleast a portion of the string.
 12. The racquet string of claim 1 inwhich the composite is longitudinally wrapped around at least a portionof the string.
 13. The racquet string of claim 1 wherein the polymericmembrane comprises fluoropolymer.
 14. The racquet string of claim 13wherein the fluoropolymer is expanded polytetrafluoroethylene.
 15. Theracquet string of claim 1 in which the composite has a thickness of lessthan about 5 percent of the string diameter.
 16. The racquet string ofclaim 1 in which the composite has a thickness of less than about 3percent of the string diameter.
 17. The racquet string of claim 1 inwhich the composite has a thickness of less than about 1 percent of thestring diameter.
 18. The racquet string of claim 1, wherein the stringcomprises natural gut.
 19. The racquet string of claim 1, wherein thestring is a mono-filament.
 20. The racquet string of claim 1, whereinthe string comprises a plurality of filaments.
 21. The racquet string ofclaim 20, wherein said filaments are of substantially the same diameter.22. A racquet string having a diameter of less than about 1.34 mm andhaving dynamic modulus of less than about 30 kN/m and a durability of atleast about
 2200. 23. A racquet string having a diameter of less thanabout 1.25 mm and having dynamic modulus of less than about 30 kN/m anda durability of at least about
 500. 24. A racquet string having adiameter of less than about 1.25 mm and having dynamic modulus of lessthan about 30 kN/m and a durability of at least about
 1000. 25. Aracquet string having a diameter of less than about 1.20 mm and havingdynamic modulus of less than about 30 kN/m and a durability of at leastabout
 500. 26. A racquet string having a diameter of less than about1.20 mm and having dynamic modulus of less than about 30 kN/m and adurability of at least about
 800. 27. The racquet string of claim 1,wherein the string comprises synthetic fibers.
 28. The racquet string ofclaim 27, wherein the synthetic fibers comprise polyamide.
 29. Theracquet string of claim 28, wherein the synthetic fibers comprise nylon.30. The racquet string of claim 27, wherein the synthetic fiberscomprise polyester.
 31. The racquet string of claim 27, wherein thesynthetic fibers comprise PEEK.
 32. A racquet comprising: a) a frame; b)a string disposed in the frame, and c) a composite comprising apolymeric membrane having at least some porosity and an adhesivedisposed within said at least some porosity, the composite covering atleast a portion of the string.
 33. The racquet of claim 32 wherein theat least some porosity is filled with the adhesive.
 34. The racquet ofclaim 33 further comprising an adhesive layer disposed upon at least onesurface of the polymeric membrane.
 35. The racquet of claim 34 whereinthe adhesive layer is discontinuous.
 36. The racquet of claim 34 whereinthe adhesive layer is continuous.
 37. The racquet of claim 32 in whichthe adhesive comprises a low temperature curable adhesive.
 38. Theracquet of claim 38 in which the low temperature adhesive is UV cured.39. The racquet of claim 37 wherein the low temperature adhesive furthercomprises at least one filler material.
 40. The racquet of claim 39wherein the at least one filler material comprises at least a materialselected from the group consisting of: ceramics, metals, metal coatedfillers, metallized fillers, inorganic oxides, carbon, pigments,lubricants and polymers.
 41. The racquet of claim 37 wherein the lowtemperature adhesive comprises at least a material selected from thegroup consisting of urethane acrylates and cationic epoxies.
 42. Theracquet of claim 32 in which the composite is helically wrapped aroundat least a portion of the string.
 43. The racquet of claim 32 in whichthe composite is longitudinally wrapped around at least a portion of thestring.
 44. The racquet of claim 34 wherein the polymeric membranecomprises a fluoropolymer.
 45. The racquet of claim 44 wherein thefluoropolymer is expanded polytetrafluoroethylene.
 46. The racquet ofclaim 32 in which the composite has a thickness of less than about 5percent of the string diameter.
 47. The racquet of claim 32 in which thecomposite has a thickness of less than about 3 percent of the stringdiameter.
 48. The racquet of claim 32 in which the composite has athickness of less than about 1 percent of the string diameter.
 49. Theracquet of claim 32, wherein the string comprises natural gut.
 50. Theracquet of claim 32 wherein the string is a mono-filament.
 51. Theracquet of claim 32, wherein the string comprises a plurality offilaments.
 52. The racquet of claim 49, wherein said filaments are ofsubstantially the same diameter.
 53. The racquet of claim 32, whereinthe string comprises synthetic fibers.
 54. The racquet of claim 53,wherein the synthetic fibers comprise polyamide.
 55. The racquet ofclaim 54, wherein the synthetic fibers comprise nylon.
 56. The racquetof claim 53, wherein the synthetic fibers comprise polyester.
 57. Theracquet string of claim 53, wherein the synthetic fibers comprise PEEK.58. A racquet comprising a frame and a string disposed within the frame,the string having a diameter of less than about 1.34 mm and havingdynamic modulus of less than about 30 kN/m and a durability of at leastabout
 2200. 59. A racquet comprising a frame and a string disposedwithin the frame, the string having a diameter of less than about 1.25mm and having dynamic modulus of less than about 30 kN/m and adurability of at least about
 500. 60. A racquet comprising a frame and astring disposed within the frame, the string having a diameter of lessthan about 1.25 mm and having dynamic modulus of less than about 30 kN/mand a durability of at least about
 1000. 61. A racquet comprising aframe and a string disposed within the frame, the string having adiameter of less than about 1.20 mm and having dynamic modulus of lessthan about 30 kN/m and a durability of at least about
 500. 62. A racquetcomprising a frame and a string disposed within the frame, the stringhaving a diameter of less than about 1.20 mm and having dynamic modulusof less than about 30 kN/m and a durability of at least about
 800. 63. Aracquet string comprising: a) a core; and b) a composite comprising apolymeric membrane having at least some porosity and an adhesivedisposed within at least some of said at least some porosity, thecomposite covering at least a portion of the core.
 64. A racquetcomprising: a) a frame; b) a string comprising a core, the stringdisposed within the frame; and c) a composite comprising a polymericmembrane having at least some porosity and an adhesive disposed withinsaid at least some porosity, the composite covering at least a portionof the core.
 65. The method of covering a racquet string comprising thesteps of: a) providing a racquet string; b) providing a polymericmembrane having at least some porosity c) filling at least some of saidat least some porosity with an adhesive to form a composite; and d)wrapping at least a portion of the string with the composite.
 66. Aracquet string comprising: a) a string; and b) a composite comprising:an expanded polytetrafluoroethylene membrane having at least someporosity, an adhesive substantially filling said at least some porosity,and a continuous adhesive layer disposed upon at least one surface ofsaid expanded polytetrafluoroethylene membrane, said composite coveringat least a portion of said string.
 67. A racquet string comprising: a) astring; and b) a composite comprising an expandedpolytetrafluoroethylene membrane having at least some porosity, anadhesive substantially filling said at least some porosity, and adiscontinuous adhesive layer disposed upon at least one surface of saidexpanded polytetrafluoroethylene membrane, said composite covering atleast a portion of said string.